Probably no reason other than the person who implemented quicksand didn't think of adding a linedef type for the sideless version at the time. You can make a sideless quicksand FOF yourself with the...

I'm not sure what you mean. To edit a sector's properties, just right-click on it in Sectors Mode. If you want the properties window to open automatically after drawing a sector, you can enable that...

Zone Builder v2.8 is out! Nothing fancy, just two bugfixes I made after the last release that I don't want to withhold any longer. The more important one is that linedef flags are no longer lost when...